In apparatus or tooling for forming end panels or shells for metal cans or plastic containers, for example, as disclosed in U.S. Pat. No. 5,042,284 of which applicant is a co-inventor, it is desirable to construct the tooling so that the shells are produced from sheet metal or aluminum having a minimum gage or thickness. On the other hand, it is necessary for each shell to have sufficient strength for withstanding a predetermined pressure within the can without deforming or buckling. It is also desirable for the tooling to provide for high volume production of the shells on either a single or multiple action press and to complete the forming of each shell at a single station in order to avoid complicated reforming operations. Commonly, an end panel or shell includes a circular center panel which is connected by a panel radius and an annular panel wall to a U-shaped countersink portion having a countersink radius. The countersink portion is connected by a tapering or frusto-conical chuckwall portion to an upper crown portion which extends outwardly to a depending peripheral lip portion.
One of the common problems encountered in producing end panels or shells is the stretching and thinning of the sheet metal when forming a small panel radius and a small countersink radius. If there is stretching and thinning of the sheet metal in these areas, the strength of the shell rapidly decreases, with the result that the shells are unacceptable for use. The stretching and thinning of the sheet metal around the panel radius and countersink radius can result from tooling which draws the chuckwall and center panel from the sheet metal.
The center panel wall and the countersink have also been formed after drawing the chuckwall, for example, as disclosed in U.S. Pat. No. 4,715,208. In this patent, the center panel is moved upwardly with the die center and panel punch after the chuckwall is formed. However, this method does not provide for a uniform countersink radius or a small panel radius or a cylindrical panel wall of maximum length, each of which is important for producing a high strength shell with a sheet material of minimum thickness. Other forms of tooling and method for producing shells are disclosed in U.S. Pat. No. 4,637,961. In this patent, the chuckwall is formed at one tooling station and then the center panel, panel wall and countersink are formed at a second tooling station.
There is also a problem of forming shells to precision dimensions and specifications when a high speed mechanical press is starting up production and the press and tooling are cool or at room temperature. That is, as the press warms up with operation, the press dynamics and the thermal expansion of the press components change so that the desired precision dimension from the top of the crown portion to the bottom of the countersink portion changes. This condition may be corrected by stopping the press after the press and tooling have arrived at operating temperature and then readjust the ram so that it bottoms at a slightly different height in order to produce shells according to specification. However, it is very undesirable to shut down a high production can producing press after it has arrived at operating temperature in order to make such an adjustment. Also when the tooling is constructed to produce a large number of shells with each press stroke, for example, 24 shells, the dynamic loading on the press may overload the press if the speed of the press is increased.